A true coilover will have a treaded shock body that will allow you to thread the perch up and down. These are really nice because the shock is usually [on the good kits anyway] valved to match the spring rates of the coil spring. This will allow for a much better ride. One benefit of coilovers are the fact that they allow you to corner balance your car to help with handling.
Coilover sleeves slide over the stock or aftermarket shock, but you can run into problems with these such as incompatibilites with the valving of the shock and the spring rate giving you a really rough, bumpy, and even sometimes uncontrolable ride. Some of these kits squeak because of the way they mount over the shocks.. I had these and switched back to Eibachs and Konis.
Regular springs and shocks are a great combo. Not usually as agressive as a coilover yet still provide great performance. The spring just rest on the shock perch that is usually not height adjustable. [Konis will allow you to move the perches up and down a little but its not threaded it just rests on a C ring.]
